Miniaturization of a camera body and image-taking (shooting) from as near position to an object as possible are requested in consumer use cameras with lenses. Therefore, an inner-focus lens is used mainly in the consumer use cameras, which corrects the movement of an image plane with the movement of a magnification lens by driving a correction lens according to cam track data without mechanically interlocking the correction lens and the magnification varying lens using a cam, and performs focusing by driving the correction lens. The cam track data instructs the movement track of the correction lens and are stored in a microcomputer.
FIG. 7 shows the structure of a conventional inner focus lens system. In the Figure, 901 denotes a front lens, which is fixed, 902 denotes a zoom lens (magnification varying lens; it is also called as a variator) that is a first lens unit. Reference numeral 903 denotes a stop, and 904 denotes a fixed lens. Reference numeral 905 denotes a focus lens as a second lens, which has a focusing function and a correcting function (that is, a compensating function) that corrects the movement of the image plane with magnification. Reference numeral 906 denotes an image-pickup surface.
The position of the focus lens 905 to form an object image on the image-pickup surface 906 is changed according to object distances even though the focal length of the lens system is not changed because the focus lens 905 has the compensating function and the focusing function in the lens system shown in FIG. 7. FIG. 8 shows cam tracks that are formed by continuously plotting the positions of the focus lens 905 to form the object image on the image-pickup surface 906 when the object distance is changed in each focal length. By selecting one from the cam tracks according to the object distance and driving the focus lens 905 along the selected cam track, variation of the magnification (zoom) with an in-focus state maintained can be performed.
In a lens system in which focusing is performed using a front lens, a focus lens is provided independently for the zoom lens, and furthermore the zoom lens and the focus lens are mechanically interlocked with a cam ring. Therefore, when an operator rotates the cam ring manually and rapidly to change the focal length, the cam ring can rotate according to the operation. Since the zoom lens and the focus lens are driven in the optical axis direction by cams formed on the cam ring, an object image does not blur due to magnification if the focus lens is located at an in-focus position.
In contrast, in the inner focus lens system, it is common to select a cam track based on the positions of the focus lens and zoom lens from the plurality of cam tracks (they are also referred to as electrical cam tracks) or information corresponding to the cam tracks (the information can be given as a function of zoom lens positions), which are stored in a memory, and perform zooming by driving the focus lens along the selected cam track.
Here, the in-focus state can be maintained by driving the focus lens along the cam track when zooming from the telephoto side to the wide-angle side because the plurality of cam tracks converge from a state in which they have some degree of intervals, as shown in FIG. 8. However, the in-focus state cannot be maintained by the same method when zooming from the wide-angle side to the telephoto side because one cam track that the focus lens should follow from the convergent point of the plurality of cam tracks cannot be determined.
A control method has been disclosed in Japanese Patent Publication No. 2,795,439, in which a focus lens is driven in an out-of-focus direction from an in-focus position using an AF evaluation value signal (sharpness signal) obtained from the high-frequency component of a video signal, and furthermore driven in an in-focus direction by changing its driving condition. This is a so-called “zigzag correcting operation”, which is a control method in which control for changing the following speed to the cam track is repeated. Thereby, the cam track that the focus lens should follow is corrected.
In addition, a method has been also disclosed in Japanese Patent Publication No. 2795439, in which the increase and decrease cycle of the sharpness signal is changed by changing the change amount of the following speed (driving condition) according to the object, focal length and depth of field to increase the accuracy of selection (determination) of the cam track that the focus lens should follow.
In the zigzag correcting operation disclosed in Japanese Patent Publication No. 2,795,439, the following speed to the cam track is changed according to the focal depth and focal length, etc. However, the AF evaluation value is changed according to not only the focusing state but also the change of object's pattern.
Therefore, to recover an error of change of the zigzag correcting operation direction (correction direction), the change amount of the following speed (correction intensity) is set according to the widening degree of the cam tracks in FIG. 8 so that, if the focus lens moves out of a right cam track that it should follow originally, it can return to the right cam track again.
Especially, in the telephoto side range in which the cam tracks disperse perfectly, if an error in determination of the cam track occurs once, it will take a long time to return the focus lens to the right cam track because of a long movement distance, and in the meantime, an image blur will occur. Therefore, by maximizing the correction intensity in the middle zoom range where the cam tracks start to disperse, the cam track is determined before the zoom position reaches the telephoto side range.
However, in the conventional control method in which the large correction intensity is set in the middle zoom range, there are the following problems.
In a case in which zooming is started from the middle zoom range where the correction intensity is large, the following speed correction corresponding to the correction intensity is performed from an in-focus state. The correction intensity in the middle zoom range is originally set so that the focus lens can be moved to an in-focus cam track according to the dispersion degree of the cam tracks. Therefore, in almost all cases where zooming is continuously performed from the wide-angle side to the middle zoom range, it is assumed that the focus lens is out of the in-focus cam track.
However, especially, in a case where zooming is started from an in-focus state in the middle zoom range, the zigzag correcting operation by the conventional control method causes an out-of-focus movement corresponding to the correction intensity to the focus lens. Although an image blur is not clearly visible in a taken-image when the object is fixed and the object distance is constant, a considerable image blur occurs when a camera work such as panning is performed or the object is moving. Because when the AF evaluation value is changed according to the change of the taken-image due to the camera work or the movement of the object, misjudgment of an in-focus direction is caused, and the misjudgment in a high correction intensity condition causes a large movement of the focus lens in an out-of-focus direction.
In addition, in a case where a low-contrast object is taken, the level of the AF evaluation value in an in-focus state is low, and the change amount of the AF evaluation value from an out-of-focus state to an in-focus state is small. However, strong correction of the focus lens to a side in which an image blur occurs according to the start of zooming causes a large image blur. In this case, if the correction direction is reversed, it is not possible to find a right cam track because an increasing amount of the AF evaluation value is small. Therefore, a situation in which zooming reaches the telephoto end with an image blur is caused.